As Kojima put it, GZ exists to help ease gamers into the full open-world experience. "The main section [of the game] is just really big," he said, "and you can have enemies attack you from 360 degrees around. As time passes, you reactions in-game will change depending on player actions. If you do a mission, you'll be tired out. You'd be exhausted after a round of airsoft, right? So let's take an hour-long break...something sort of like that. So to keep people from thinking 'This isn't MGS!', we'll limit the amount available to you at first and set a static time and weather pattern. So you'll get to play this open-world, highly free MGS in those conditions."
In case there was still any confusion as to the time line for Ground Zeroes and The Phantom Pain, Kojima confirmed that GZ will take place in 1975, one year after the events of Peace Walker. The bulk of the game, The Phantom Pain, will take place in 1984. You will more than likely need to play GZ in order to understand the story and motivations for Big Boss in The Phantom Pain. Naturally, the events in this game should lead into the start of the series, Metal Gear.